1. Field of the Invention
This invention relates to a semiconductor device including a test circuit for evaluating the characteristics of components of the semiconductor device, and more particularly to an improvement of such test circuit.
2. Description of the Related Art
Integrated-circuit chips contain a good assortment of components. A MOS field effect transistor (hereinafter called "MOSFET") is one of such component. It is essential to know the precise characteristics of the MOSFET components of the integrated circuit. Consequently, in many instances, test circuits for measuring the characteristics of individual components of an integrated circuit are formed on a semiconductor substrate such as wafer independently.
The characteristics of MOSFET are tested to evaluate the electrical characteristics of the components. Therefore, it is preferable to perform all the measurement electrically and to obtain consistent data.
However it is impossible for conventional test circuits to electrically measure essential characteristics such as the effective channel length and gate length necessary for controlling the effective channel length. For instance, the gate length has to be measured optically based on a measurement principle which differs from the principles of electrical measurement. Therefore, the data that are obtained by the different measurement principles are sometimes inconsistent. It also takes time to perform the measurements.
Conventional test circuits have difficulty precisely measuring the characteristics of the MOSFET by a simple method. Specifically, different test circuits have to be used to measure the effective channel length and the sheet resistance of the MOSFET, respectively, resulting in test circuits that occupy a large space on the wafer.
FIG. 6 of the accompanying drawings show an example of a test circuit MOSFET. With MOSFET, the effective channel length should be precisely measured, since it is as important as the threshold voltage.
The test circuit 100 includes at least two MOSFETs 10a, 10b whose gates 12a, 12b have the same width W but has different lengths L. The sources, drains and gates of these MOSFETs 10a, 10b are respectively connected to metal wirings and measuring electrodes 16, 18, 20a, 20b via contact holes 14.
A computer-controlled measuring unit (not shown) contacts probes to the measuring electrodes 16, 18, 20a, 20b of the test circuit 100. For example a predetermined voltage is applied to the gates of MOSFETs 10a and 10b to measure the current across the source and the drain. The effective channel length is then calculated based on the current and applied voltage characteristics of MOSFETs 10a and 10b.
FIG. 7 shows a test circuit 110 for measuring the sheet resistance of MOSFET. A gate 12 of MOSFET is usually made of polycrystalline silicon, for example, similar to a resistor component. Therefore, it is necessary to measure the resistance of the MOSFET gate as the sheet resistance (specified in units of ohms per square) after excluding the influence of the parasitic resistance. For this purpose, the test circuit 110 includes an element 22 for measuring the sheet resistance. The element 22 is connected to four measuring electrodes 24, 26, 28, 30 via contact holes 14. A computer-controlled measuring unit (not shown) contacts its probes to these measuring electrodes 24, 26, 28, 30, measuring the sheet resistance of the element 22.
The test circuits 100, 110 are applicable to electrically measure the effective channel length and the sheet resistance of MOSFETs. A typical application of these test circuits is for the evaluation of trial products using the measured results.
In practice, such test circuits 100, 110 are also applied to evaluate mass-produced articles. In such case, it is also required to obtain data such as the gate length for controlling the effective channel length, difference between the gate length and the effective channel length, and the lateral diffusion length of the source and drain.
However these data cannot be obtained from the electrical measurements of the test circuits 100, 110. For example, the gate length has to be optically measured by using laser beams.
The optical measurement should be carried out for an exposed gate region 12 during the fabrication of the test cirucit 100 under the conditions quite different from the electrical measurement. Further, the gate length obtained by the optical measurement, which is different from the principle of electrical measurement, does not always match the effective channel length and the sheet reistance obtained by the electrical measurement. Therefore, the obtained data are not always reliable enough to evaluate MOSFETs.
There has been a great demand to make compact test circuits and to reduce the number of measuring electrodes to effectively use the measuring probes, so that the test circuits are appliable to evaluation of the mass-produced articles without adversely affecting highly integrated circuits.
Conventionally, two test circuits 100, 110 are required to measure the important data such as the effective channel length and the sheet resistance as shown in FIGS. 6 and 7. This means that these test circuits not only occupy rather large spaces on the semiconductor substrate but also need a number of measuring electrodes.
Specifically, the test circuit 110 for measuring the sheet resistance is large and needs many electrodes.